ASTRONOMERS have identified the smallest known black hole 鈥� a dark void in space-time only about 20 kilometres wide. They have also found four unusually heavy neutron stars, and together these objects are plugging a mysterious gap in the range of masses of dead stars that roam our galaxy.
The discovery of these 鈥渕issing links鈥� has delighted astronomers. 鈥淚t鈥檚 reassuring because there is no physical reason that these things shouldn鈥檛 be there,鈥� says David Nice of Princeton University in New Jersey. 鈥淭his has been quite a puzzle.鈥�
Both neutron stars and black holes form when very massive stars blow up in supernova explosions at the end of their lives. Before his work on the atomic bomb, the American physicist Robert Oppenheimer showed in the 1930s that the size of the remaining core is critical to what happens next.
Advertisement
According to today鈥檚 calculations, if the core weighs between about 1.4 and 3 solar masses it will form a neutron star: a superdense ball of neutrons typically 10 kilometres wide with a solid shell of iron nuclei. If the core of the star is heavier than about 3 solar masses, however, no known force can prevent gravity crushing it to a point, forming a black hole from which nothing 鈥� not even light 鈥� can escape.
Thanks to the latest orbiting X-ray telescopes, which detect searing hot gas raining down on neutron stars and black holes, astronomers have now found dozens of them in our galaxy. But until now all the neutron stars have seemed strangely similar, weighing close to 1.35 solar masses. Then there was a gap, with the lightest black holes weighing in at about 5 solar masses (see Figure). 鈥淣o one had any idea why this mass gap existed,鈥� says Dawn Gelino of the University of California at San Diego.
Now Gelino and Thomas Harrison of New Mexico State University in Las Cruces think they鈥檝e found a black hole that bridges this gap. They used telescopes at Apache Point Observatory in New Mexico and Lick Observatory in California to look at an object discovered in 1992 called J0422+32, which lies about 8000 light years away in the constellation Perseus.
Their observations suggest that the system contains a black hole weighing between 3 and 5 solar masses, making it the lightest black hole on record. 鈥淭his finding is the first to show that a black hole can have a mass of 4 solar masses, or even lower,鈥� says Gelino. Her conclusions, which will appear in The Astrophysical Journal, are at .
In separate work, Nice and his colleagues have discovered four neutron stars with possible masses ranging from 1.4 to 3 solar masses, which also fill in the strange mass gap. However, he suspects that when they formed they weighed 1.35 solar masses but have since grown fatter as they sucked more material from nearby stars. That leaves another unsolved mystery 鈥� why are all newborn neutron stars exactly the same size? 鈥淲e really don鈥檛 know why,鈥� says Nice.
David Helfand of Columbia University in New York says we will need a deeper understanding of supernova explosions to explain these stellar remains. Maybe it鈥檚 not just the mass of a collapsing stellar core that determines its final fate, he says. Perhaps some other factors, such as the spin of the core or its magnetic field, are also involved. 鈥淲e don鈥檛 really understand what makes a supernova decide to make a neutron star or a black hole,鈥� he says.
A top priority is to find far more of these dead stars. With sensitive X-ray telescopes like NASA鈥檚 Chandra X-ray Observatory and the European Space Agency鈥檚 XMM-Newton telescope, both launched in 1999, astronomers hope to discover and weigh dozens more neutron stars and black holes, and figure out exactly how they formed. 鈥淚f we are to fully understand stellar evolution, we need to understand their entire lives, from birth until death,鈥� says Gelino.